The Semantic Kernel uses the following grammar to parse prompt templates:
# BNF parsed by TemplateTokenizer
[template] ::= "" | [block] | [block] [template]
[block] ::= [sk-block] | [text-block]
[sk-block] ::= "{{" [variable] "}}" | "{{" [value] "}}" | "{{" [function-call] "}}"
[text-block] ::= [any-char] | [any-char] [text-block]
[any-char] ::= any char
# BNF parsed by CodeTokenizer:
[template] ::= "" | [variable] " " [template] | [value] " " [template] | [function-call] " " [template]
[variable] ::= "$" [valid-name]
[value] ::= "'" [text] "'" | '"' [text] '"'
[function-call] ::= [function-id] | [function-id] [parameter]
[parameter] ::= [variable] | [value]
# BNF parsed by dedicated blocks
[function-id] ::= [valid-name] | [valid-name] "." [valid-name]
[valid-name] ::= [valid-symbol] | [valid-symbol] [valid-name]
[valid-symbol] ::= [letter] | [digit] | "_"
[letter] ::= "a" | "b" ... | "z" | "A" | "B" ... | "Z"
[digit] ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"